a) Field of the Invention
The invention is directed to an arrangement for the generation of short-wavelength radiation based on a hot plasma generated by gas discharge and to a method for the production of coolant-carrying electrode housings for the gas discharge, in particular for a radiation source for the generation of extreme ultraviolet (EUV) radiation in the wavelength range of 11 nm to 14 nm.
b) Description of the Related Art
As structures of integrated circuits on chips become increasingly smaller in the future, radiation of increasingly shorter wavelength will be needed in the semiconductor industry for exposure of these structures. Lithography machines with excimer lasers whose shortest wavelength is reached at 157 nm and in which transmission optics or catadioptric systems are employed are currently in use. Based on Moorer's law, new radiation sources with even shorter wavelengths must be made available in the future in order to increase imaging resolution in the lithographic process for semiconductor chip fabrication.
Since there are no available transmission optics for these new radiation sources with wavelengths below 157 nm, reflection optics must be used. However, as is well known, these reflection optics have a very limited numerical aperture. This results in a decreased resolution of the optical systems which can only be compensated by a further reduction in wavelength.
There are several known techniques suitable for the generation of EUV radiation (in the wavelength range from 11 nm to 14 nm), of which the generation of radiation from laser-induced plasma and from gas discharge plasmas shows the greatest potential. There are, in turn, several concepts for gas discharge plasmas, e.g., plasma focus, capillary discharge, hollow cathode discharge, and Z pinch discharge. In the latter technique, an especially great effort has been directed toward cooling the electrodes. However, the solutions developed for this can also be applied to the other gas discharge techniques.
The prior art solutions for electrode cooling are basically tied to a cooling circuit in which, for the most part, cooling channels with rib structures are used in the electrode bodies.
U.S. Pat. No. 6,815,900 B2, for example, discloses a radiation source for the generation of EUV radiation based on a gas discharge plasma and describes optimized concentric electrode housings for achieving a high average radiation output and long-term stability. The gas discharge takes place between a collar-shaped anode and cathode in the interior of the electrode housing. Cavities with ribs, porous material or capillary structures (so-called heat pipe arrangements) through which a coolant flows are provided in the walls of the electrode housings.
US 2004/0071267 A1 discloses a plasma focus radiation source for the generation of EUV radiation which uses lithium vapor and which likewise has a coaxial anode-cathode configuration. In order to reduce erosion and increase the lifetime of the electrodes, a heat pipe cooling arrangement is provided in addition to the combined thermal radiation cooling and thermal conduction cooling so that the electrode tips are kept below the melting temperature even though these electrode tips comprise high-melting tungsten. The principle of liquid evaporation is used in the heated area of the heat pipe and that of condensation in a cold area of the heat pipe. The liquid is returned via a wick. Because of the high latent evaporation heat from the vaporization and condensation of lithium (vaporization heat of 21 kJ/g), it is possible to transfer a heat load of about 5 kW without high mass flow rates.
Further, US 2004/0160155 A1 discloses a gas discharge EUV source which suppresses debris exiting from the plasma by means of a metal halogen gas generating a metal halide with the debris exiting the plasma. The source has a special anode comprising differentially doped ceramic material (e.g., silicon carbide or alumina) containing boron nitride or a metal oxide (such as SiO or TiO2) as dopant so as to be electrically conductive in a first region and thermally conductive in a second region, the first region being associated with the electrode surface. This electrode is then cooled through a hollow interior having two coolant channels or porous metal which defines coolant passages.
All of the above-described solutions for electrode cooling have the disadvantage of a comparatively high cost of production, particularly when cooling is effected by bundles of capillary structures or by porous material which exceeds the cost and compactness of simple cooling mechanisms (e.g., cooling channels provided with ribs) many times over. Other disadvantages include the impossibility of a monolithic construction, the complexity, and the relatively large space requirement for integrating the special structures for increasing the surface in the electrodes.
Since the complexity, the dimensions and, above all, the cost of a radiation source of this type according to the gas discharge concept described above determine the ultimate success or failure of the radiation sources when used in semiconductor lithography, an attempt must be made to develop the individual components (e.g., the electrodes with cooling arrangements) at a lower technological and financial cost with the same or higher efficiency (particularly with respect to lifetime) compared to current highly developed technology.